Sticking with the ol' use what Big Ed Turner provided principle, the standard perches were always gonna be pressed into service. Had to get some 3/8" UNF screwed rod to transfer the position to the seat pan, tried 10mm studding but it's too big! Less than 0.5mm up and it's too much, that's accurate 3/8" bore tube they're using there.
Locking the rods with nuts either side set the position fair and square, and fixed the length equally. Marking the seat pan was plain sailing after that.